a) Field of the Invention
This invention relates to a control method for a vehicle engine arranged in association with a fluid coupling having a clutch, which is suited for use in controlling the engine at the time of a deceleration of the vehicle.
b) Description of the Related Art
Automatic transmissions equipped with a clutch (hereinafter called a "damper clutch") in their torque converters have been provided to date.
Such automatic transmissions are each provided with the torque converter connected to an engine, the damper clutch incorporated in the torque converter to permit solid connection of the torque converter between an inlet side and an outlet side thereof, and a control unit for the torque converter and the damper clutch.
The above control unit is constructed of non-slip connection control means for controlling the damper clutch in a solid connection state and non-direct connection range control means for controlling the damper clutch in a state not directly connected.
By operating the vehicle with the damper clutch directly connected by the non-slip direct connection control means under predetermined conditions, fluid frictional engagement in the torque converter can therefore be replaced by mechanical frictional engagement by the damper clutch so that the fuel consumption can be improved.
The non-slip direct connection control for the damper clutch is designed to be performed in the steady operation state that the throttle position is equal to or greater than a predetermined angle and the turbine speed of the torque converter is equal to or greater than a predetermined value.
With respect to direct connection control of a damper clutch, deceleration-time direct connection control which is performed during a deceleration of an automotive vehicle has also been proposed in addition to the conventional direct connection control which is conducted in a steady operation state.
If direct connection control is performed during a deceleration of an automotive vehicle as described above, the feeding of fuel can be reduced owing to inertia force on a side of driving wheels while preventing a drop in the engine speed. The fuel consumption can therefore be improved.
Deceleration-time direct connection control however involves a problem as will be described next.
If deceleration-time direct connection control is performed by fully closing a throttle valve from the running state that a damper clutch is in a direct connection control state or in a slipped direct connection control state (i.e., the state of direct connection in a slipped direct connection range so that the connection is accompanied by a certain degree of slip), a change in torque of an engine caused by the closure of the throttle valve is transmitted directly to a turbine shaft as an output side of a torque converter so that a shock or a judder occurs on a wheel-driving side.
As a measure for overcoming such a problem, it may be contemplated of once bringing the damper clutch into a non-direct connection state and initiating the deceleration-time direct connection control after allowing the torque converter to absorb the change in torque. Adoption of this measure however leads to another problem which will be described next.
Since the feeding of fuel to the engine is reduced during the deceleration-time direct connection control, the hydraulic pressure for holding the damper clutch in the deceleration-time direct connection state is set at a very low level. Even if it is attempted to change the damper clutch from the non-direct connection state to the direct connection state by this low holding hydraulic pressure, the low holding hydraulic pressure cannot supersede a release pressure still remaining on a release side (i.e., the side on which feeding of a hydraulic pressure changes the damper clutch into the non-direct connection state) so that the damper clutch is not promptly brought into the deceleration-time direct connection state. The engine speed therefore drops and feeding of fuel is therefore resumed on a side of an engine control unit. It is therefore impossible to improve the fuel consumption.
If a high apply pressure (i.e., a pressure applied to a side on which feeding of a hydraulic pressure directly connects the damper clutch) sufficient to supersede the release pressure is caused to act, the damper clutch undergoes rapid engagement so that a shock occurs in a drive system.
When braked hard on a low .mu. road (low friction coefficient road) such as a snow-covered road or in a like situation, the brake on the side of the driving wheels is transmitted to the engine to brake revolutions of the engine provided that deceleration-time connection control (including slipped connection to non-slip connection) is being performed. It is therefore necessary to release the direct connection state of the damper clutch so that the engine speed can be maintained at a predetermined level. Application of a high apply pressure to the damper clutch, however, may lead to a delay in the release of the direct connection state, thereby involving the potential problem that the engine speed is lowered to cause an engine stall.
This is also attributed to the circumstance that the capacity of an oil pump of the transmission is dependent on the engine speed and a drop in the engine speed at the time of a deceleration reduces the capacity of the oil pump of the transmission, thereby failing to promptly release the direct connection of the damper clutch.
Further, the delivery rate of the oil pump is also lowered when release of the direct connection under the deceleration-time connection control and the shift control take place at the same time. This lowered delivery rate is also responsible.